As is known in the art, Radio Frequency (RF) couplers are four port or input/output RF devices and have a wide range of applications. One type of coupler is a quadrature coupler shown in FIGS. 1A and 1B to include: a pair of strip conductors SC1, SC2 physically separated one from the other by a dielectric board B1 (FIG. 1B) and disposed between a pair of ground plane conductors GP1, GP2 (FIG. 1B) formed on the upper surfaces of a corresponding one of a pair of dielectric boards B2 and B3, (FIG. 2B) respectively, as shown. More particularly, each one of the a pair of strip conductors SC1, SC2 has an input port I1, I2 (FIG. 1A) respectively, coupled to a pair of output ports O1, O2, (FIG. 1A) respectively, through an electromagnetic coupling region CR (FIGS. 1A, 1B). The electromagnetic coupling region CR is a region where a portion of the strip conductors SR1 SR2, in this configuration, vertically overlay one another and are separated by a vertical gap G (FIG. 1B). It is in this electromagnetic coupling region CR that radio frequency energy passing through the strip conductors SC1, SC2 is coupled between the pair of strip conductors SC1, SC2 by electromagnetically passing through the gap G. It is noted that the opposing ends of strip conductor SC1 are connected to the input port I1 (FIG. 1A) and the output port O1 (FIG. 1A) respectively, while the opposing ends of the strip conductor SC 2 are connected to the input port I2 (FIG. 1A) and the output port O2 (FIG. 1A), respectively as shown. More particularly, one portion of an input signal fed input port I1 passes to output port O1 and another portion of the input signal at input port I1 is coupled by the electromagnetic coupling region CR to both output ports O1 and O2; output port O2 typically being connected to a matched load, not shown. The above described coupler is sometimes referred to as an overlay coupler; another type of coupler is a broadside coupler (FIGS. 1C and 1D) where instead of the electromagnetic coupling region CR being a pair of overlaying strip conductors, as in FIGS. 1A and 1B, the pair of strip conductors SC1, SC2 (FIGS. 1C and 1D) where instead of the electromagnetic coupling region CR being a pair of overlaying strip conductors, as in FIGS. 1A and 1B, the pair of strip conductors SC1, SC2 (FIGS. 1C and 1D) are on the same surface of a common dielectric board Ba (FIG. 1D) and the portions of the strip conductors SC1, SC2 in the electromagnetic coupling region CR are in a side by side arrangement and are separated by a horizontal gap G (FIGS. 1C and 1D).Thus, while here again the pair of strip conductors SC1, SC2 are physically separated one from the other by a dielectric boards Ba and B1 (FIGS. 1C and 1D), radio frequency energy is electromagnetically coupled between the strip conductors SC1, SC2 by electromagnet energy passing between them through the gap G. Thus, here again, it is in this electromagnetic coupling region CR that radio frequency energy passing through the strip conductors SC1, SC2 is electromagnetically coupled between the pair of strip conductors SC1, SC2.
It is desirable that the surface area occupied by the coupler be minimized. Several couplers are discussed in the following papers: Design of Compact Multilevel Folded-Line RF Couplers by Settaluri et al., IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 12, DECEMBER 1999, pages 2331-2339; and COMPACT MULTI-LEVEL FOLDED COUPLED LINE RF COUPLERS, Settaluri et al., 1999 IEEE MTT-S Digest pages 1721-1724.